1. Field of the Invention
The present invention relates to a method of manufacturing porous ceramic bodies, and more particularly to a method of manufacturing porous ceramic bodies with gradient of porosity, in which a gradient that is continuous to a pore size and porosity is precisely controlled in a simple way, to thus be applicable for the porous ceramic bodies.
2. Description of the Related Art
The higher a plant grows, the smaller the diameter of the vessel of the plant becomes. At the highest portion of the plant, the diameter of the vessel is reduced to a nanoscale extent in size, to thus show powerful capillary action. In addition, when water evaporates due to evapotranspiration, a pressure difference occurs between the roots of the plant and the other parts thereof such as flowers, leaves and stems, which is also a big factor to help the roots take up water.
Meanwhile, the animal's circulatory system also forms a similar structure to that of the plant. This is interpreted as a phenomenon that biological solutions move in tiny capillary vessels by the Starling equation. In addition, when cells absorb biological solutions, capillary suction occurs, to thereby enable tiny capillary vessels to take up the biological solutions.
FIG. 1 shows FESEM (Field Emission Scanning Electron Microscope) images with a planar view and a cross-sectional view of a chicken bone. FIG. 2 shows a micro-Computed Tomography (CT) image of the chicken bone of FIG. 1. In fact, bones of creatures have a structure that external and internal pores are interconnected with one another via pathways, and such a structure can be found in the FIGS. 1 and 2 images. However, unlike birds, the bones of human beings have a gradient porous structure with gradient of porosity in which the outside of each bone has a high density and the inside of each bone has a low density in the form of a sponge.
In general, since porous ceramics have excellent thermal shock resistance and excellent insulating properties, they can complement problems such as wear of bones and a difference in intensity thereof that are exposed at the time of using existing bone implants. In the case of the existing bone implants, surfaces of ceramics are oxidized to improve biocompatibility, but this oxidizing process is only a surface treatment. As a result, although it is the limit that cells do not grow to the inside of the implants, porous ceramics exhibit an excellent performance at the time of inducing bone regeneration because blood vessels or bone cells are easily harmonized into a porous structure.
However, unless the porous structure is precisely controlled, the mechanical strength becomes weak. Thus, the optimal conditions should be found by controlling porosity and a gradient of pores. Ceramics whose porosity is well-controlled prevent progression of cracks, to thus have high intensity and excellent resistance in comparison with ordinary materials.
A method of mixing and sintering powder or fiber with a difference in density or a tape-casting method is available as a conventional method of manufacturing ceramic bodies with gradient of porosity. However, it is not easy to control processing steps that are applied for the method of manufacturing ceramic bodies, and it is limited to obtain a continuous gradient.
A technology of implementing gradient of porosity by sequentially stacking porous plates with respectively different porosities was disclosed in U.S. Patent Application Publication No. 2005/0100578. In this case, boundary-surfaces that are formed by tying plates may be separated and chipped away from the sequentially stacked plates. In addition, a technology of forming a porous structure by mixing ceramic powders of different sizes and then melting some powders through a heat treatment process was disclosed in U.S. Pat. No. 7,699,903. However, it was still insufficient to form a continuous gradient of porosity.
Meanwhile, U.S. Patent Application Publication No. 2011/0097259 discloses a technology of mixing sponge and ceramic slurry and then burning the sponge during performing a heat treatment process. However, it has a disadvantage that it is difficult to control the sponge precisely on a nanoscale basis. U.S. Patent Application Publication No. 2009/0074832 proposed implant devices of gradient of porosity that was manufactured through an electrospinning method. However, there were defectives that polymer implant devices offered a low intensity and the electrospinning method was not suitable for mass production. U.S. Patent Application Publication No. 2006/0121609 proposed a method of forming pores through a sublimation process of moisture during lyophilizing (that is, freeze-drying) a slurry. However, as recommended in the U.S. Patent Application Publication No. 2006/0121609, only those who have a well-trained skill were able to control pores to be formed. Thus, it had a disadvantage of leading to poor reproducibility and disadvantageous mass production.
In addition, U.S. Patent Application Publication No. 2004/0247855 or Japanese Patent No. 07-062470 proposed a method of manufacturing ceramic foam with gradient of porosity in heterogeneous catalysis through a centrifugation method. However, there was a difficulty that an accurate technology for rheology and mass transfer of slurry during drying in order to obtain a desired shape was needed. This technology was far from applications for bone implants.
Korean Laid-open Patent Publication No. 2005-0095395 proposed a method of manufacturing ceramic bodies with gradient of porosity, which includes the steps of: controlling a mixture ratio of several types of ceramics and powder for controlling pores, to thus obtain the mixed powder; adding binders to the mixed powder to thus manufacture a number of loads having respectively different porosity to then perform incorporation; and undergoing an extrusion, fat removal, and sintering process. However, gradient of porosity is not continuous, a process is cumbersome, and materials of different properties are attached each other to then be sintered again, thereby causing cracks to be easily formed on junction surfaces.
In addition, Japanese Laid-open Patent Publication No. 2001-206787 disclosed a method of manufacturing porous sintered bodies formed of calcium phosphate series whose porosity is distributed in a continuous gradient of 5-85%. Here, according to a technology of manufacturing the porous sintered bodies, powder of calcium phosphate series is dispersed in an organic solvent to thus manufacture slurry, calcium phosphate bodies are manufactured by crosslinked polymerization by adding a foaming agent to the slurry, the calcium phosphate bodies are put and sintered one upon another several times or are made to rotate, to thereby make porosity lowered as it goes from the center of the calcium phosphate bodies to the outside thereof. However, this method uses organic solvents and other additives, to thus cause impurities that may remain after the end of the process harmful to human bodies, make the process complicated and make it difficult to precisely control gradient of porosity.